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NITROGEN TRANSFORMATIONS AND NO 3 − REMOVAL AT A SOIL–STREAM INTERFACE: A STABLE ISOTOPE APPROACH
Author(s) -
Ostrom Nathaniel E.,
Hedin Lars O.,
von Fischer Joseph C.,
Robertson G. Philip
Publication year - 2002
Publication title -
ecological applications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.864
H-Index - 213
eISSN - 1939-5582
pISSN - 1051-0761
DOI - 10.1890/1051-0761(2002)012[1027:ntanra]2.0.co;2
Subject(s) - biogeochemical cycle , denitrification , groundwater , dissolved organic carbon , riparian zone , hydrology (agriculture) , transect , environmental science , hyporheic zone , stable isotope ratio , environmental chemistry , geology , nitrogen , ecology , chemistry , oceanography , physics , organic chemistry , biology , geotechnical engineering , quantum mechanics , habitat
The natural removal of NO 3 − by denitrification within riparian zones of streams and rivers is an area of considerable interest owing to its potential to minimize the impacts of excess anthropogenic loadings. In this study we utilize natural variations in stable N isotopic compositions of NO 3 − and NH 4 + within a transect of shallow wells extending 4 m inland from Smith Creek, a southwestern Michigan stream, to provide insight into microbial processes and the extent of NO 3 − removal within a soil–stream interface. Within this region three water masses with unique biogeochemical characteristics intersect: a shallow flow rich in NH 4 + and dissolved organic carbon (DOC), a deep groundwater mass rich in NO 3 − but depleted in DOC, and stream water low in NO 3 − , NH 4 + , and DOC. N isotope values for NO 3 − within the well transect were highly variable (−7.7–34.1‰) and reflected intense microbial activity within this narrow region. Isotopic variation was primarily controlled by upwelling of deep groundwater near the stream and partial loss of NO 3 − via denitrification that was dependent upon a supply of DOC from shallow groundwater. Quantitative estimates of the fraction of NO 3 − removed due to denitrification within the soil–stream interface were obtained from N isotope data using a modified Rayleigh equation. Conservative estimates of NO 3 − removal range from 0% to 86%. In conjunction with measurements of hydrological flows within the sampling wells we provide a novel estimate of NO 3 − removal based only on natural abundance isotope measurements. NO 3 − removal was found to vary from undetectable levels to 123 mmol·L −1 ·m −2 ·d −1 for the narrow region of the soil–stream interface in which deep and shallow groundwater intersect.